The present invention relates to a foldable lithium element and anode assemblies in an electrochemical cell for use with medical devices. This invention further relates to methods of making such anode assemblies.
Electrochemical cells having thin planar anode assemblies have found particular applications in the medical field for use with heart pacemakers and other medical devices. General teachings concerning such cells may be found, for example, in U.S. Pat. No. 5,209,994 (hereinafter ""994), assigned to the assignee of the present invention. The ""994 cell includes a container of electrically conductive material which serves as a cathode current collector. The anode assembly of the cell includes a lithium element formed from two lithium halves which are pressed together with an anode current collector therebetween. The anode current collector extends to the exterior of the cell with use of an insulator which insulates a lead connected thereto from electrical contact with the container. The container is filled with a cathode material which is in operative contact with the exposed surfaces of the lithium element of the anode assembly. Similarly, the cathode material is in operative contact with the container. For enhanced performance of the cell, the opposed, major lateral surfaces (i.e., the xe2x80x9coperative surfacesxe2x80x9d) of the lithium element may be coated with a film of electron donor material. More specifically, ""994 describes this donor material as being a polymeric organic donor material such as poly (2-vinylpyridine). Such donor materials and application techniques for such materials are more fully described in U.S. Pat. No. 4,182,798.
In operation, a chemical reaction between the lithium element and the cathode material in the container causes excess electrons to flow into the current collector. A chemical reaction between the cathode material and the container causes the container to be positively charged. The resulting voltage differential can be used to power a device. To prevent the cell from short-circuiting, the anode current collector is electrically insulated from the cathodic container and from the cathode material which fills the container. As noted above, an insulator (i.e., a feedthrough) allows the anode current collector to extend to the exterior of the container without making electrical contact with the cathodic container. Additionally, the anode current collector is protected from contact with the cathode material by the seal formed by cohesion between the two lithium halves between which the collector is embedded.
In a conventional method for forming an anode assembly, two lithium pre-cut elements are positioned on opposite sides of an anode current collector. An insulated portion of the anode current collector which insulates the collector from the cathodic container is also typically positioned between the two lithium elements. The subassembly is then placed within two mold sections and is pressed together with a suitable force. The current collector and the insulator portion are sealed between the two lithium elements with a portion of the current collector (i.e., the lead) extending from the pressed together lithium elements for electrical connection of the electrochemical cell to a medical device.
Conventionally, the lithium halves are roughened, e.g., brushed, to enhance cohesion between the pre-cut lithium halves. Cohesion of the lithium halves sealing the anode current collector therein is necessary to prevent the cathode material from reaching the anode current collector and rendering the electrochemical cell inoperative. As such, techniques of enhancing such cohesion are needed.
In electrochemical cells, anode assemblies using lithium elements have been found to provide relatively small and efficient cells, particularly in conjunction with cathode materials, such as iodine or thionylchloride. However, costs associated with using pre-cut lithium halves to form such anode assemblies is of concern. Lithium has continuously been increasing in price as have labor costs associated with each pre-cut element. As such, there is a need for anode assembly configurations which at least hold the line on such costs.
Table 1 below lists U.S. Patents that describe electrochemical cells having thin plate anodes:
All patents listed in Table 1 above and elsewhere herein are hereby incorporated by reference in their respective entirety. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments and Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art with respect to lithium elements and anode assemblies in electrochemical cells. One such problem is obtaining satisfactory cohesion between lithium elements in an anode assembly. To enhance cohesion, it has been typical practice to roughen the facing surfaces of the lithium plates by xe2x80x9cbrushing,xe2x80x9d such as with an abrasive material before the plates are pressed together. Brushing leaves fresh, unoxidized lithium material exposed which coheres relatively well. However, this brushing step adds labor and time to the cost of manufacturing the resulting cell.
A further problem with the present two lithium element anode assemblies is that the stamping of two separate lithium elements requires a relatively significant amount of lithium material, time and labor, and generates excessive waste material. An arrangement that reduces scrap resulting from lithium element production, without impairing the performance of the cell, would offer significant advantage.
Various embodiments of the present invention have the object of solving at least one of the foregoing problems. Further, an embodiment that requires only a single lithium element, thereby eliminating manufacturing time and labor, would reduce the cost of manufacturing a cell. In addition, an embodiment that minimizes the amount of scrap in production of the lithium elements is also advantageous. Still further, an embodiment that does not require brushing prior to pressing would reduce the time and cost of manufacturing.
In comparison to known lithium elements and anode assemblies, various embodiments of the present invention may provide one or more of the following advantages: eliminating one of the two separate lithium elements, thereby reducing the time and labor required to produce the second lithium element; enhancing the flow of the lithium material during pressing which enhances the cohesion for adequate sealing of the anode current collector, thereby reducing the need for xe2x80x9cbrushingxe2x80x9d the facing surfaces of the lithium plates prior to pressing the plates together; and minimizing the amount of scrap material in the production of the lithium elements, thereby reducing the cost of the element and thus cost of the cell.
Some embodiments of the invention include one or more of the following features: a folded lithium element; a folded lithium element having first and second sections in which at least a portion of the first section lies adjacent to at least a portion of the second section to receive a current collector therebetween; a folded lithium element having first and second sections where surface areas of the sections are substantially equivalent; and a folded lithium element having first and second sections where a surface area of the first section is less than a surface area of the second section.